The invention relates to optical data storage. More specifically, the invention relates to a method for reading data from an optical storage medium such as a DVD or a CD.
“Read/write” optical discs include optical discs that allow data to be written only once and optical discs that allow data to be written many times. A DVD+RW disc is a type of read/write disc that allows data to be written many times.
When writing new data to a read/write disc, it is desirable not to create a frequency or phase discontinuity between the data being written (“new” data) and data previously written (“old” data). A read/write drive might not be able to tolerate such discontinuities during readback of the old data and the new data. During readback, the discontinuities can cause problems for read clocks and data recovery circuitry. Consequently, the discontinuities can render portions of the read/write disc effectively unreadable by the read/write drive.
The problems resulting from these discontinuities may be overcome by the use of “edit gaps” (also known as “splice areas” and “buffer zones”). Edit gaps are spaces that separate data blocks. An edit gap provides a margin of error so that old data is not overwritten by new adjacent data. However, the use of edit gaps has its drawbacks. Storage capacity of the read/write disc is reduced because data is not stored in the edit gaps.
Certain DVD read/write drives are capable of performing bit-accurate or linkless editing, whereby new blocks of data are written with negligible frequency or phase discontinuity with respect to adjacent previously written blocks of data. This bit accurate editing is accomplished by exploiting accurate timing information embossed in a high frequency wobbled groove in the read/write disc. The bit-accurate editing eliminates the need for edit gaps.
During bit-accurate editing, non-negligible phase discontinuities between old data and new data may be created due to tracking offsets while writing, asymmetric signal levels, etc. A phase discontinuity is illustrated in FIG. 3. Marks to the left of a block boundary BB represent old data, and marks (in solid) to the right of the block boundary BB represent new data. Marks in phantom reflect the correct phase of the new data.
If a phase discontinuity is encountered during readback, the read clock usually isn't able to track the new phase immediately. Instead, the read clock slews to the new phase. However, errors may occur while the read clock is slewing to the new phase. This may result in bit slipping, where the data recovery is misaligned by one or more bits. It may be a while before the read clock becomes realigned with the data having the new phase. Consequently, one or more lines of error correction capability might be lost.
The loss of error correction capability can reduce margins during readback. Error correction capability allows drive manufacturers to reduce tolerances of their drives. Any readback errors that occur as a result of lower mechanical tolerances, lower quality of optics, etc., can be corrected by performing error correction. Thus, error correction capability allows the cost of the drive to be reduced. Loosing error correction capability, on the other hand, forces the manufacturers to rely on higher tolerances. Hence, loss of error correction capability can result in higher manufacturing costs.
The loss of error correction capability can also reduce data access time. If an error occurs during readback of a data block and that error cannot be corrected, the block will be re-read. Each re-read reduces data access time.
Other formats such as CD typically overcome the phase discontinuity problems by including a preamble containing sync fields that allow the read clock to be synchronized with the new data block. For ROM compatibility reasons, the DVD+RW linkless format does not contain such sync fields.
It is therefore desirable to mitigate the disruptive effects of phase discontinuities during readback of read/write optical discs.